1. Field of the Invention
The present invention relates to a receiving device for receiving a CDMA (Code Division Multiple Access) signal in a CDMA communication system and a channel estimator included in the receiving device for correcting the deterioration of the CDMA signal ascribable to multipath propagation.
2. Description of the Background Art
Today, extended researches are under way on a CDMA communication system capable of enhancing the efficient use of limited frequencies available with mobile communication. In the CDMA communication system, multiplexed interference signals other than a desired wave and received from the other transmitting stations and interference signals ascribable to multipaths are dealt with in the same manner as thermal noise in a spreading/despreading process. With this scheme, the CDMA system allows a plurality of transmitting stations proportional in number to a process gain to share the same frequency band. A spread spectrum communication system essential with the CDMA system is disclosed in, e.g., U.S. Pat. No. 5,228,056 to Donald Schilling, assigned to InterDigital Technology Corporation and entitled xe2x80x9cSYNCHRONOUS SPREAD SPECTRUM COMMUNICATION SYSTEM AND METHODxe2x80x9d. This document teaches that messages of different kinds are spread by use of spreading codes of different kinds and then despread to turn out the original messages.
In the next generation mobile communications art, there is an increasing demand for adaptability to multimedia radio transmission and greater subscriber capacity. DS (Direct Sequence)-CDMA is a radio access system meeting the above demand and is a promising candidate for IMT (International Mobile Telecommunications)-2000/FPLMTS (Future Public Land Mobile Telecommunication Systems) and other third generation systems because of its high frequency utilization efficiency. IMT-2000 is discussed in, e.g., xe2x80x9cIMT-2000 Radio System Standardization Processxe2x80x9d, the Proceedings of the Institute of Electronics, and Information Communication Engineers of Japan, May 1998, pp.473-478.
In a DS-CDMA system, while a plurality of stations share the same frequency, each of them spreads a signal to be transmitted by use of a particular code. A receiving station is capable of separating a desired signal by despreading a received signal by use of the same code as the associated transmitting station. In practice, however, interference noise occurs due to cross-correlation between codes assigned to the individual stations, intersymbol interference, and the autocorrelation of multipath of a desired station, limiting the subscriber capacity. An interference canceller technology is available for reducing the influence of the interference noise. An interference canceller installed in a base station allows the cell capacity to be increased and allows the transmission power required of a mobile terminal to be reduced. In addition, the interference canceller enhances accuracy with which the path of an acquired desired signal is tracked.
Some receiving devices for CDMA communication, for example, each includes a correlator or a matched filter. The problem with this type of conventional receiving device is that interference ascribable to transmitting stations other than desired one increases with an increase in the number of subscribers, aggravating the demodulating ability of the receiving device to a critical degree.
In light of the above, some different interference canceller systems have heretofore been proposed, but each of them has some problems left unsolved, as follows. A parallel interference canceller, for example, estimates and cancels interference in parallel and thereby reduces a delay. This type of interference canceller, however, cannot improve the interference cancelling ability of a receiving device because it estimates a great number of transmitting stations at the same time. To improve the interference cancelling ability, parallel interference cancellers with a weighting scheme have been proposed and generally classified into two kinds. However, one kind of cancellers with a weighting scheme cannot improve the ability to a noticeable degree. The other kind of cancellers with a weighting scheme is not satisfactory in practice because a weighting coefficient for implementing the optimal performance depends on the number of transmitting stations and received power.
By contrast, a serial interference canceller, or SIC (Successive Interference Canceller), capable of cancelling interference serially has an advantage that its ability increases with an increase in the number of stages, i.e., the number of times of demodulation. The serial interference canceller is disclosed in, e.g., Young C Yoon et al. xe2x80x9cA Spread-Spectrum Multi-Access System with a Cascade of Co-Channel Interference Cancellers for Multipath Fading Channelsxe2x80x9d, IEEE Second International Symposium on Spread Spectrum Techniques and Applications (ISSSTA ""92), Nov. 29-Dec. 2, 1992. This document discusses the characteristic of the serial interference canceller in a multipath fading environment.
However, the problem with the serial interference canceller is that it cannot perform the estimation and cancellation of interference with two or more transmitting stations at the same time. As a result, delay noticeably increases with an increase in the number of transmitting stations or the number of stages. Assuming thirty transmitting stations and five stages, then the delay exceeds 450 symbols, i.e., 150xc3x973 (delay of about three symbols for a delay cancelling unit)=450 even in the ideal case. Such an amount of delay is not practical. The delay time would further increase if a delay time particular to a channel estimator were taken into account.
For example, the delay time of a conventional channel estimator is more than half a slot (0.625 ms/2) for a station. To reduce the delay time, there has been proposed a hybrid interference canceller (HIC) which is the combination of a parallel interference canceller featuring short delay and a successive interference canceller featuring high performance. For the HIC, receivers situated at users"" stations are divided into groups which are connected in series. The receivers are arranged in parallel in each group. This kind of scheme, however, cannot achieve a satisfactory ability unless the number of groups and therefore the delay is increased.
Assume that an ICS (Interference Canceller System) is used to increase the subscriber capacity. Then, although demodulation itself is guaranteed by the ICS, searchers arranged around the ICS for acquiring synchronization and DLLs (Delay Lock Loops) for executing synchronous tracking have their performance degraded due to an increase in the number of users constantly holding communication, i.e., an increase in interference. This is also true with the receipt of an access channel; to insure accurate receipt, the transmission power of an access channel must be greater than conventional and aggravates interference.
It is therefore an object of the present invention to provide a receiving device for CDMA communication featuring an improved interference cancelling ability and capable of reducing, even when the subscriber capacity is increased, the deterioration of the abilities of DLLs, searchers and access channel receiver without aggravating receipt delay.
In accordance with the present invention, a receiving device for CDMA communication includes a receiving circuit for receiving signals for CDMA sent from a plurality of transmitting stations to thereby output received signals. A first group of interference cancellation stage estimates signals sent from the transmitting stations from the received signals by using despreading codes corresponding to spreading codes respectively assigned to the transmitting stations. The first group of interference cancellation stage includes N (integer greater than 2 inclusive) interference cancelling units (ICUs) each for estimating an interstation interference ascribable to interference between the despreading codes, and estimates interstation interferences in parallel, and cancels the interstation interferences to thereby estimate the signals sent from the N transmitting stations in parallel. A second group of interference cancellation stage estimates, from signals from which the interstation interferences have been cancelled by the first group of interference cancellation stage, interstation interferences in parallel with M (integer greater than 2 inclusive) ICUs, cancels the interstation interferences to thereby estimate signals sent from the M transmitting stations in parallel, and outputs signals free from the interstation interferences. A plurality of interference cancellation stages each having the first group of interference cancellation stage and second group of interference cancellation stage are serially connected to construct a hybrid interference cancelling unit. The interference cancellation stages each estimates interstation interferences while the ICUs of the interference cancellation stage output the signals sent from the individual transmitting stations.
Also, in accordance with the present invention, a channel estimator includes an interference cancelling circuit for estimating signals sent from the transmitting stations from the received signals by use of despreading codes respectively corresponding to spreading codes respectively assigned to the transmitting stations. The interference cancelling circuit estimates interstation interferences ascribable to interference between the spreading codes and/or interference between the despreading codes, and cancels the interstation interferences contained in the received signals to thereby output an interference-free signal. A demodulating circuit demodulates the received signals to thereby output demodulated signals. A tracking circuit executes synchronous tracking on the basis of the demodulated signals to thereby execute tracking control over chip clocks to be used by the interference cancelling circuit.